Grating-outcoupled surface-emitting lasers

ABSTRACT

Laser diodes are formed with an outcoupling grating between two separate distributed Bragg reflectors. The devices have gain regions located between the reflector gratings for pumping the active region. The outcoupling grating couples light out of the waveguide normal to the surface if the grating spacings are equal to an integer number of wavelengths of the light within the cavity. If the gratings are not such an integer number, the light is coupled out of the cavity off the normal.

CROSS-REFERENCE TO OTHER APPLICATION

[0001] This application claims priority from 60/200,603 Filed Apr. 28,2000; 60/200,454 Filed Apr. 28, 2000; 60/209,822 Filed Jun. 6, 2000;60/230,534 Filed Sep. 1, 2000; and 60/235,090 Filed Sep. 25, 2000, filedApr. 28, 2000, which is hereby incorporated by reference.

BACKGROUND AND SUMMARY OF THE INVENTION

[0002] The present application relates to laser diodes, and moreparticularly to extracting light from a waveguide and coupling thatlight to a fiber or other device.

BACKGROUND

[0003] Transmission of light through waveguides has been pursued formany types of communications applications. Light signals offer manypotential advantages over electronic signals. Light sources are commonlycreated from semiconductor devices, and include semiconductor devicessuch as LEDs (Light Emitting Diodes) and LDs (Laser Diodes).

[0004] Optical fiber is the most commonly used transmission medium forlight signals. A single fiber is capable of carrying several differentmodulated signals within it at one time. For instance, wavelengthdivision multiplexing divides the used bandwidth of the fiber intodifferent channels (each channel containing a small range ofwavelengths) and thus transmits several different wavelengths (orsignals) of light at once. Using such a system requires sources for thedifferent wavelengths. More wavelengths on the fiber require moresources to be coupled to the fiber.

[0005] Efficient coupling of light into a fiber is simplified if thelaser beam has a cross sectional profile that matches the profile of thefiber mode(s). Efficient use of light for communications requires thatthe light have high temporal coherence. Efficient coupling of light tomonomode guides requires spatial coherence. Spatial coherence requiresthe laser to operate in a single lateral and transverse mode. Temporalcoherence requires the laser to operate in a single longitudinal modeand implies a very narrow bandwidth, or range of wavelengths.

[0006] The most coherent semiconductor lasers use resonators based ongrating feedback rather than Fabry-Perot resonators with reflective endfacets. Distributed feedback (DFB) lasers use a Bragg reflective gratingcovering the entire pumped length of the laser. An alternative to DFBlasers is the use of distributed Bragg reflectors (DBRs) located outsidethe pumped region.

[0007] In conventional DFB and DBR lasers, light is removed through anend facet and the output beams have dimensions entirely controlled bythe vertical (i.e., normal to the surface) (x) and lateral (y) size andthe composition of the guiding structure. Such output beams aretypically have too great a divergence for effective coupling to opticalfibers, or for other applications requiring beams with low divergenceangles.

[0008] Beam dimensions (in at least one direction) larger than thatavailable from laser facets may be obtained by using a Bragg grating tocouple light out of the waveguide normal (or at certain fixed angles) tothe waveguide surface. So called second order Bragg gratings have aperiod equal to the wavelength of light of the guided mode. The secondgrating order of such a grating reflects some of the light back in thewaveguide plane while the first order couples some of the light normalto the plane. So called first order (Bragg) gratings have a period equalto one half the wavelength of light in the guided mode, reflect light inthe waveguide plane, and do not couple light out of the waveguide.First, second, and third order (etc.) gratings are sometimes referred toas being in resonance. A non-resonant grating couples light out of thewaveguide at an angle to the normal and does not reflect any light inthe waveguide plane.

[0009] U.S. Pat. No. 5,970,081 to Hirayama et al. appears to show alaser with a distributed feedback (DFB) grating of second order orhigher that claims to obtain a Gaussian shaped output beam by narrowingthe waveguide or using a chirped grating at the outcoupling portion.They do not seem to recognize that by so doing the resonant wavelengthof the grating is altered along the length of the narrowing or chirping.This would be expected to result in an output which will fan in anglealong the longitudinal direction rather than produce a simple Gaussianintensity variation emitted normal to the plane as claimed. They do notdefine the beam shape in the lateral direction. In all versions theychoose second order outcoupling gratings which, absent a narrowingwaveguide or chirp, would emit light perpendicular to the surface of thelaser waveguide.

[0010] U.S. Pat. No. 4,006,432 to Streifer et al. appears to show agrating out-coupled surface emitting DFB laser. The grating period maybe chosen to be either resonant or not.

[0011] A paper by Bedford, Luo, and Fallahi titled Bow-TieSurface-Emitting Lasers (IEEE Photonics Technology Letters, Vol. 12, No.8, Aug. 2000) appears to show a DBR laser with curved second ordergrating at the ends to couple light out of the waveguide. The samegratings are used for outcoupling and for reflecting the light withinthe waveguide. They mention the use of non-resonant gratings inconjunction with reflector gratings if emission at other than thedirection normal the waveguide plane is desired. The paper appears toshow a flared resonator region which allows symmetric outcoupling fromboth ends of the laser. This facilitates two outputs that are coherentwith one another, with emission in the same direction. Such a device ismeant to alleviate the complications of controlling the relative phasebetween the two emitters.

[0012] The Tiberio article (Facetless Bragg reflector surface-emittingAlGaAs/GaAs lasers . . . , J. Vac. Sci. Technol., B9(6), 1991) appearsto show a surface emitting laser diode that uses first order reflectivegratings and either second order (or non-resonant) gratings foroutcoupling. Thus, depending on the chosen grating period, theoutcoupled beam can be normal or at an angle to the surface.

[0013] U.S. Pat. No. 6,064,783 to Congden appears to show a DBR laserwith a grating assisted waveguide coupler that couples light from thelaser waveguide to a parallel fiber-like glass waveguide for latercoupling to a fiber. Several different lasers are coupled to similarfiber-like glass waveguides in the figures. The fiber axis is parallelto the laser waveguides. This reference mentions that this model iseasily attached to a fiber through “butt coupling.” The grating acts asa Quasi Phase Matching element to couple the light from the laserwaveguide to the fiber-like glass waveguide.

[0014] The optical and electronic properties of a semiconductor dependon the crystal structure of the device, which has led to investigativework into fabricating artificial structures or superlattices. Bytailoring the crystal structure of a device during its fabrication, theoptical and electronic properties can be controlled. The crystalstructures of such devices may be controlled, for instance, by molecularbeam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD).Such techniques are capable of monolayer control (˜5 angstrom) over thechemical composition of a crystal.

[0015] Other commonly used heterostructures are quantum wells, in whicha single layer of one semiconductor is sandwiched between two layers ofa larger bandgap material. Strain is produced by using an epitaxiallayer with a different lattice constant than the substrate. This strainhas a dramatic effect on the properties of the optical system. Amongother things, it can allow bandgap tunability, reduced thresholdcurrent, and improved laser reliability.

[0016] Strain can also allow laser emission to have tailoredpolarization. By using appropriate strain, one can produce lightpredominantly polarized as TE, or TM.

[0017] Grating-Outcoupled Surface-Emitting Lasers

[0018] This application discloses a laser source. In a preferredembodiment, the source is a laser diode and has an active region withdistributed Bragg reflectors at either end to reflect light within thecavity, and an outcoupling grating in the center of the device whichcouples light out of the cavity. On either side of the outcouplinggrating is a gain region with electrical contacts for supplying current.

[0019] The outcoupling grating positioned between the reflectors allowsthe light to be coupled out with less loss than other laser diodes. Thelight that is not outcoupled stays within the cavity. This design hasthe advantage that the outcoupler size can be limited to much shorterlengths than other designs, on the order of about 10 microns in thepreferred embodiment. In laser diodes where the outcoupler is outsidethe cavity, it is very difficult to couple out 100% of the light in soshort a distance. Light that is not coupled out decreases deviceefficiency. The surface emitting outcoupling laser diode can alsocontrol the outcoupled beam shape by varying the shape of theoutcoupling grating.

[0020] Our approach avoids the problems cited in OCG devices because itshows innovative grating emitter structures located within the lasercavity and independent of the type of reflectors. these allow efficientlaser operation. We teach how to shape the area and vary the propertiesof the grating emitters to produce desired output beams and he;pstabilize the laser mode to enhance spatial and temporal coherence. Wealso show that embodiments that produce several wavelengths forefficient coupling and multiplexing to broad band optical fibers. ourstructures also allow integration with many devices including broad bandmodulators, switches, and isolators.

[0021] This design makes it easier to limit the length of theoutcoupling grating to short lengths, on the order of about 10 microns.In designs where the outcoupler is outside the DBR laser region, itbecomes very difficult to outcouple 100% of the light in such a shortdistance. The light that is not coupled out is wasted and decreasesdevice efficiency.

[0022] The disclosed innovations, in various embodiments, provide one ormore of at least the following advantages:

[0023] low cost;

[0024] device testing at the wafer level;

[0025] emission at all wavelengths from 0.6-2.0 microns with existingand common material systems, with greater ranges possible;

[0026] emission is easily extended to any wavelength as new materialsystems mature and/or are developed;

[0027] low drive currents;

[0028] higher power capability than existing VCSELs;

[0029] high efficiency;

[0030] direct replacement for VCSELs;

[0031] easily coupled to multi-mode and single-mode fibers.

BRIEF DESCRIPTION OF THE DRAWING

[0032] The disclosed inventions will be described with reference to theaccompanying drawings, which show important sample embodiments of theinvention and which are incorporated in the specification hereof byreference, wherein:

[0033]FIG. 1a shows side view of an innovative DBR laser.

[0034]FIG. 1b shows a top view of an innovative DBR laser.

[0035]FIG. 2a shows a side view of crossed DBR lasers.

[0036]FIG. 2b shows a top view of crossed DBR lasers.

[0037]FIG. 2c shows a close up of crossed outcoupling gratings.

[0038]FIG. 3 shows a side view of a DBR laser with a reflectiveundercoating to reflect laser light.

[0039]FIG. 4a shows a top view of a DBR with flared or tapering gainregions.

[0040]FIG. 4b shows a top view of crossed DBRs each with flared ortapered gain regions.

[0041]FIG. 5a shows a side view of a laser diode having a DBR at one endand a cleaved facet at the other end.

[0042]FIG. 5b shows a top view of crossed lasers, using both DBRs andreflecting facets.

[0043]FIG. 5c shows a top view of a laser diode using cleaved facets.

[0044]FIG. 6 shows a laser diode using cleaved facets and a reflectivelayer beneath part of the waveguide.

[0045]FIG. 7a shows a top view of four crossed DBR lasers eachoutcoupling light from the same outcoupling element.

[0046]FIG. 7b shows a close up of the crossed gratings for the lasersystem of FIG. 7a.

[0047]FIG. 8 shows a circuit diagram of integrated elements with thepresently disclosed laser system.

[0048]FIG. 9 shows optical waveguides routing light from the laser toother elements.

[0049]FIG. 10 shows a possible configuration for integrated elementswith a laser diode.

[0050]FIG. 11 shows another possible configuration for integrating addedelements to the present innovations.

[0051]FIG. 12 shows another embodiment of the present application.

[0052]FIG. 13 shows another embodiment of the present application.

[0053]FIG. 14 shows a DBR laser with thinned quantum wells beneath theoutcoupling grating and beneath the DBRs.

[0054]FIG. 15 shows another embodiment of the present application.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0055] The numerous innovative teachings of the present application willbe described with particular reference to the presently preferredembodiment (by way of example, and not of limitation).

[0056] First order outcoupling gratings and second order or higheroutcoupling gratings are both used in at least some embodiments of thepresent innovations. In the present application, first order DBR refersto a distributed Bragg reflector grating that reflects light within thewaveguide in first order for feedback. A second order DBR grating willoutcouple light in first order, and feedback light in second order.

[0057] In several variations in this application, second order feedbackgratings (which couple light out in first order) are used. In sucharrangements, the feedback grating depth or strength is varied in the yand z directions so that both the loss and the feedback from the gratinghelp to stabilize the laser mode. For example, the first order lateralmode will be stabilized if the grating strength is varied so that thefeed back varies like a Gaussian. This is accomplished by forming thegrating so that its strength varies as

1−exp [−(y/ω)²]

[0058] where y is the direction parallel with the feedback gratingsurface and perpendicular to the cavity length, with the origin taken tobe at the center of the out-coupling grating, and omega is half the ygrating dimension.

[0059] First order outcoupling gratings are gratings which couple lightout of the waveguide plane in first order but may or may not satisfy thein-plane Bragg condition for second or higher order Bragg reflection.Such gratings may be designed to create no second or higher orderreflections which feedback into the laser mode. In these variationswhich use such out-coupling gratings with no in-plane feedback, thegratings cause no destabilizing feedback into the laser mode and aremechanically and electrically isolated from the structure used to formand pump the resonant laser cavity. Thus, the length and position of theoutput grating can be chosen to suit the needs of the application forwhich the laser is designed. The grating periods required foroutcoupling, with and without in-plane reflections, are summarized in“Surface Emitting Semiconductor Lasers and Arrays,” G. A. Evans and J.M. Hammer, Eds., Academic Press, 1993, which is hereby incorporated byreference.

[0060] In general, second and higher order feedback gratings can resultin some outcoupling. However, these are less preferred in the context ofthe present application since such higher order interactions are lessefficient.

[0061] The outcoupling angle of the gratings in the innovative systemsherein disclosed is measured as an angle from the normal to the surfaceof the outcoupling grating. Resonant outcoupling occurs when theoutcoupling grating has a period that is equal to an integer number ofwavelengths of the light in the cavity. A grating with period equal tothe wavelength of light in the laser cavity will outcouple some lightnormal to the laser plane and reflect some light in-plane in secondorder. This means the light exits the grating parallel or nearlyparallel to the normal. Outcoupling of light off the normal occurs whenthe grating is not an integer number of guide wavelengths, and in such acase the light exits the grating at an angle from the normal. This angledepends on the difference between the guide wavelength and the gratingperiod. Varying the wavelength of light or the outcoupling gratingperiod can therefore have great effect on the angle of outcoupled light.The out-coupling grating length, longitudinal position, and the outputangles may therefore be chosen over a large range of values. The gratingmay also be shaped to achieve an output beam of a desired cross section.This is valuable for coupling the output light into fibers of differentcross sectional size or at different angles than exactly or nearlynormal. All of these “off normal” parameters may be varied without fearof significant mode destabilization or disruption of laser coherence. Inthe case of exactly or near normal outcoupling, there can be significantin-plane reflection that may (or may not) adversely affect theperformance of the laser.

[0062]FIG. 1a shows a cross sectional view of a preferred embodiment,taken to show the x-z plane. It should be understood that the featuresin the several figures may not be to exact scale because of the largedifferences in dimension between the various structures illustrated.

[0063] Layers 3, 4, 5, and 6 are grown on a substrate 2 by known means.Each of these layers may comprise a number of sub-layers. Beneath thesubstrate is the n contact layer 14. The substrate may comprise a thicklayer of n-type semiconductor with a top layer of similar n-typematerial interposed beneath layer 3. This is frequently called then-cladding or n-clad. The n-clad will have a refractive index below thatof layer 3. Layer 3 is the active and guiding layer usually containingthe junction between p- and n-type semiconductor materials. It maycomprise, for example, a sequence of one or more barrier layersalternating with quantum well layers. Layer 4 is a p-type clad layer andhas lower refractive index than layer 3. Layer 5 may be a multi-layerincluding a p-clad material chosen to enable good contact to 6 which isthe p-metallic contact. Layer 14 provides the other electrical contactfor the laser. There are many sequences of possible layers forsemiconductor lasers and amplifiers, and the present innovations are notlimited to the structures recited here. For example, a structure with ap-type rather than an n-type substrate (and all the necessaryalterations to accommodate a change from p- to n-type materials and viceversa) is within the contemplation of the present application.

[0064] Gratings 7 are surface relief DBR gratings chosen to reflectlight in the +/−z direction to form the laser cavity. (Note that thesegratings can be buried structures within the device, and the term“surface relief” does not require the grating be on the surface of thedevice after processing.) The laser mode will be a standing wave whichmay be considered to be formed by two waves one flowing in the +zdirection, the other in the −z direction. First order DBR gratings arepreferred, but second or higher order gratings are also possible. TheDBR grating depth and length and the thickness of layer 4 are chosen toprovide the desired feedback as known in the art.

[0065] The reflector gratings can be given added functionality byvarying their grating strength or amplitude in both the y (lateral)direction and the z (cavity) direction. Variation of the gratingstrength in the lateral direction will impart to the cavity mode light aGaussian shape, allowing for more of the optical energy of the emittedlight to be coupled into a circular mode, such as a fiber. Variation ofthe grating strength in the z direction can improve the suppression ofunwanted longitudinal modes on either side of the desired longitudinalmode. the degree to which the unwanted modes are suppressed is calledthe side-mode suppression ratio.

[0066] The outcoupling grating 8 (sometimes referred to herein as OCgrating, or OCG) is a surface relief grating with period chosen tocouple light at desired angles from the grating plane. It is located atan aperture on the surface of the device. In a preferred embodiment, theoutcoupling gratings are about 10 microns wide. The outcoupling gratingmay be shaped to control the shape of the emitted beam. The gratingdepth and thickness of the p-clad layer 9 in the vicinity of the grating8 are chosen to provide the desired degree of outcoupling and to controlbeam shape. A window or aperture 10 in layers 5 and 6 is provided toallow unobstructed emission of light, and the size and shape of theoutcoupling grating is matched to the mode of the fiber to which itcouples light (in one embodiment). Because of the two standing waves inthe cavity and reflection from the grating, the outcoupling gratingsimultaneously emits four different light beams, two above and two belowthe grating plane. These are depicted by dashed arrows. In the case ofnormal outcoupling of the laser light, the two top lobes are combinedinto a single beam, as are the two bottom lobes of emitted light.

[0067] In one embodiment, the outcoupled light is emitted normal to thesurface, since one primary goal is to couple this light into a fiber.When light is emitted normal to the surface, the two top emitted beamsare combined into a single beam, and likewise with the downward emittedbeams.

[0068] Off normal emissions and slightly off normal emissions are alsovery useful. For example, changing the angle of entry to a fiber byseveral degrees has minimal impact on the coupling efficiency yet allowsthe use of an off resonance grating which minimizes undesired feedbackinto the laser. A larger angle might be desirable to send light toanother detector to monitor the laser.

[0069]FIG. 1b shows a top view of a single grating outcoupled DBR laser.The outcoupling grating 8 is located at an outcoupling aperture withinthe envelop of the gain region. On either end of the laser are locateddistributed Bragg reflectors 7 for providing feedback into the cavity.Of course, cleaved facets may also be used instead of reflectorgratings, with highly reflective coatings applied to reflect the light,as shown in later embodiments. With either DBR reflectors or coatedfacets, the reflectivity of one or both ends can be varied to allowlight to escape the cavity for other purposes.

[0070] Another embodiment will be discussed with reference to FIGS. 2aand 2 b. In this variation, crossed out-coupling gratings are locatedwithin the cavities of two (or more) semiconductor lasers arranged atangles to one another and located on a common substrate. In oneembodiment, two lasers are used and are positioned at 90 degrees fromone another, but more lasers are of course possible—see FIG. 7a forexample. The shape and strength of the two gratings are chosen toproduce desirable properties in the out-coupled light. Their periods areindividually chosen to suit the desired application, such as to controloutcoupling angle, or to couple out different wavelengths.

[0071]FIG. 2a shows a side view of the crossed grating outcoupled DBRlasers. The structure when seen from the side is similar to that ofFIG. 1. Elements that are unique to the laser running in the z-directionare labeled with a z suffix, and elements unique to the laser running inthe y-direction are labeled with a y suffix.

[0072] Referring to FIG. 2b, a top view, two crossed DBR lasers are at90 degrees to one another. Each laser has its own set of reflectorgratings 7 y, 7 z at either end, and both lasers have their ownoutcoupling grating 8 y, 8 z positioned at a common location between thereflector gratings. (In the preferred embodiment, the outcouplingaperture is located at the center of the laser, but this is notnecessary.) On either side of the out-coupling gratings are the pumpedregions of the lasers. (Note that in this variation, the two gainregions of a single laser are discontinuous, having different parts oneither side of the outcoupling grating. Other possible embodimentsinclude a single gain region with an outcoupling grating outside thegain region but between the reflector gratings, or even a singlecontinuous gain region that spans the outcoupling grating, havingportions on both sides.) The two out-coupling gratings are located atthe same place, and the superposition of the two gratings forms avirtual grating with an effective period at an angle of about 45 degreesif the grating periods are about the same fore each laser.

[0073] The reflector grating periods are chosen to internally reflectthe proper wavelength of light. The reflectivity of the DBR is very highat the Bragg condition, and drops off rapidly when the wavelength isdifferent than the Bragg condition wavelength. This allows thewavelength of the output light to be controlled by controlling theperiod of the DBR gratings.

[0074] Referring again to FIG. 2a, in the case of crossed lasers, across section taken parallel to the x-y plane would be similar withlayers noted with y subscripts in place of z subscripts.

[0075] Gratings 8 y and 8 z are surface relief outcoupling gratings withperiods chosen to couple light at desired angles relative to the grating(y-z) plane. As shown in the figure, the gratings can be shaped tocontrol the profile of the outcoupled beam. A circular profile for thegrating is indicated (a more complicated profile would be optimal forfiber coupling), but any other useful shape can be produced, dependingon the application. The grating depth along with the thicknesses andcompositions of the epitaxial structure of the laser are chosen toprovide the desired degree of out-coupling.

[0076] For each laser, four beams are emitted because of the left andright running waves that form the standing wave mode of the laser(unless the light is outcoupled perpendicular to the device). Two beamssymmetrically angled around the normal will emerge above the gratingplane and pass through the window 10. Similarly, two beams will bedirected towards the substrate below. (Note the epi layers aretransparent and this transparency can be made use of to couple light outthrough the bottom of the device. In such a case, a reflector is placedon the top of the device to direct the top emitted light back into thelaser or out the bottom.) In some designs, the grating may be blazed toallow light to be outcoupled to the right or left of normal as well.

[0077] When two or more lasers are combined in this way, the crossedoutcoupling gratings each polarize the light which they outcouple. Inthe case of two crossed gratings at 90 degrees, the two beams of lightwill be polarized at 90 degrees with respect to one another. Couplinglight into a fiber with two orthogonal polarizations is required forpumping Raman amplifiers.

[0078]FIG. 2c shows a close up of the outcoupling gratings of FIGS. 2aand 2 b. The periods, Ly and Lz, of the superimposed two gratings neednot be identical. The OCG periods will depend both on the wavelength oflight in the cavity (which in turn depends on the periods of the DBRgratings at either end of the cavity) and on the desired outcouplingangle for the emitted beam. By choosing the two gratings to havedifferent OC angles, spatial separation is possible, as may be desiredby particular applications. In still another embodiment, the laser lightfrom the devices is emitted normal to the surface, so that bothwavelengths of light can be coupled into a fiber through the sameaperture or location on the device.

[0079] By choosing a non-resonant spacing for the outcoupling gratingperiod (i.e., a distance between grating lines that is not an integermultiple of the wavelength of light within the cavity) the output beamsare emitted non-normal to the surface. This is useful in applicationswhere, for example, the fiber into which the light is to be coupled isat an angle relative to the out-coupling grating.

[0080] The choice of normal or off normal outcoupling angles can haveother advantages. For example, when two or more different wavelengths oflight are coupled out of the OC gratings, all wavelengths can be coupledinto the same fiber or separated as desired by varying the output anglesof the individual gratings. For example, the individual outcouplinggratings for two crossed devices could each couple different wavelengthsof light out normal to the surface to couple two different wavelengthsinto a single fiber mode. This is particularly applicable in the crossedgrating out-coupled lasers, discussed further below. If, for somereason, only one wavelength is needed in the fiber, the light from theother device can be emitted off normal so as to not couple into thefiber. The non fiber coupled light could be deflected to a detector, forexample. Regardless of the particular use, the choice of outcouplingangles is an advantage to a laser device, and the present applicationshows how different wavelengths from different sources can beselectively combined into a single region for coupling, or separatedinto different regions.

[0081] The shape and strength (i.e., the depth) of the OC gratings arechosen to produce desirable properties in the out-coupled light. Theperiods of all OC gratings can be individually chosen according to theneeds of that particular laser and the application. For example, the twocrossed OC gratings of FIG. 2b can be chosen to outcouple differentwavelengths of light, allowing the two lasers of the crossed laserconfiguration to have different wavelengths, one in the z-direction,another in the y-direction. This of course extrapolates to highernumbers of lasers. Additionally, the two outcoupling gratings (and thedifferent laser diodes themselves) can be chosen to emit the samewavelengths (for example, by making their feedback grating periods thesame) allowing additional power and polarization variety in theoutcoupled beam(s).

[0082] The basic idea can be extended to include a multiplicity oflasers radially arranged around a set of gratings oriented to outcouplelight independently for each laser. This allows many wavelengths oflight to be generated by merely choosing a different period for the pairof DBRs for each individual laser. The OC gratings can couple this lightinto a single spatial region (for example, to couple several wavelengthsof light into a fiber for DWDM applications), or it can couple thedifferent wavelengths out of the devices at different angles.

[0083] Referring still to FIG. 2b, which shows crossed lasers accordingto a preferred embodiment, if the Bragg reflector gratings are chosen tohave the same period in both the y-direction laser and the z-directionlaser so that both lasers operate at the same wavelength, and if thecrossed OC grating period Lz is the same as Ly, the superposition of thetwo gratings at right angles results in a virtual grating with aneffective period angle of about 45 degrees (if both grating periods arethe same). In this case the possible coupling between the y and z laserscan be avoided if the gain regions use quantum wells with compressivestrain and thus favor TE mode operation. The virtual grating at 45degrees will not efficiently reflect TE modes and therefore will notcouple the y and z lasers. On the other hand, the use of tensile strainin the quantum well favors TM modes, and may result in enough couplingto either lock the y and z lasers together into a single coherentsource, or provide significant cross-talk and other interactions betweenthe two lasers. Thus, the disclosed approach can choose the nature ofthe output beams to be either a combined single frequency coherentsource, or two output beams with two independent wavelengths, or twobeams with independent wavelengths but with a controlled amount ofcrosstalk between them.

[0084] The reflector grating periods for the pair of lasers can be thesame, which provides additional power and polarization variety.Alternatively, the grating periods can be different, resulting in twodifferent wavelengths of light being outcoupled. This latterconfiguration can couple light of different wavelengths out at the sameangle for coupling light of different wavelengths into the same fiber,saving the cost of implementing a combiner for this function. Forexample, if the two lasers have different feedback grating periods, theywill each generate a different wavelength of light. But both lasers canemit their light normal to the surface of their respective outcouplinggrating by choosing each individual outcoupling grating to couple thenecessary wavelength of light out normal to the surface.

[0085] The size of the grating output aperture can be adjusted foroptimum coupling to single or to multi-mode fiber. Likewise, theefficiency of the output element (be it a grating or other element, suchas a beam splitter or holographic optical element) can be adjusted byadding a layer of dielectric material to the outcoupling region. Ifoutcoupling efficiency is too high, a high threshold current is requiredto lase because of the quantity of photons escaping the cavity. With areflective layer atop the outcoupling grating, some of the light isreflected and continues to oscillate within the cavity. This has theeffect of marginally decreasing the required current for lasing. Addinga dielectric layer (for example, nitride, or a dielectric stack, or evena reflective metallic layer) to the outcoupling location thereforecontrols outcoupling loss and decreases the required threshold current.

[0086] In any configuration of the present application where one or moregratings are located and superimposed on one another, the separategratings can be individually formed by conventional means (includingmultiple exposures and etches to form the pattern) or can be merged intoa single element using a holographic optical element (HOE). The opticalrequirements of the gratings can be calculated, and a HOE can bedesigned that accomplishes these required optical functions. Oncedesigned, such a HOE can be patterned for lithography, and can thereforebe fabricated in fewer process steps than it would take to create themultiple gratings separately. For example, multiple divergent beams canbe captured and coupled into a single fiber with HOEs.

[0087] In another embodiment, shown in FIG. 3, a reflecting surface isplaced beneath the outcoupling gratings. This surface 11 reflects thetwo lower lobes of emitted light. This reflective layer can be made froma metallic substance or other material reflective to the necessarywavelengths of light, or it might be a reflective grating formed in thedevice, for example. By coupling the lower lobes back up and out of theOC grating, greater power is coupled out of the laser and may becaptured by a fiber or other device, such as a detector to monitor thelight produced by the device. A space 12 is shown in the substrate ofthe figure, but the same reflective surface can be placed with thesubstrate intact.

[0088] Another embodiment is shown in FIG. 4a, which shows a method toincrease the lateral width of the gain regions at the outcouplinggrating while maintaining a single-transverse mode. This is accomplishedby using a single mode waveguide in the gain region that connects to atapered gain region. The taper angle is related to the divergence of thefundamental mode of the single mode waveguide. In the preferredembodiment, the tapered regions have a laterally varying currentcontact, such as a gaussian contact to stabilize the modes in thetapered device.

[0089] The embodiment shown in FIG. 4a has a tapered gain region 13. Thegain region in this sample has a straight portion as well. Differentcontacts are used in the preferred embodiment, pumping the differentregions with increasing current as necessary. The tapered gain regionensures a wide-spatially-coherent mode, and avoids the restriction onthe lateral (y) dimension imposed by the requirement of single lateralmode operation. A wide lateral mode allows the width of the output beamto be set by the width of the grating. The grating area can take adesired shape to match the needs of various applications. For example,circular, elliptical, or Gaussian beams can be produced.

[0090]FIG. 4b shows the tapered gain regions used with multiple crossedgrating outcoupled lasers. This embodiment shows two crossed GO (gratingoutcoupled) lasers at 90 degrees to one another. DBR gratings 7 y, 7 z,create the cavity as in other embodiments discussed above. The pumpedregions of the lasers in this variation have a flared section (labeledwith the f suffix), being wider closer to the OC gratings. In thisexample, one laser has an outcoupling grating that is circular in shape,while the other laser has an outcoupling grating that is rectangular.Using a tapered gain region, outcoupled beams of a greater range ofsizes can be produced, as the gain region can be made to whatever widthis necessary to accommodate (or fully take advantage of) the size of theOC grating. Tapered gain regions also increase the total amount ofemitted power from the device.

[0091] Another embodiment is shown in FIG. 5a. At one end of the laser,the DBR has been replaced by a reflective end facet 7 y. There are stilltwo gain region portions, separated by the OC grating in thisembodiment. The other end of the laser has a DBR 7 yB, the period ofwhich determines the wavelength that will be stable in the cavity.

[0092] A top view of a crossed GO laser system is shown in FIG. 5b. Inthis variation, the laser running in the z-direction (horizontal in thefigure) has reflective end facets on both ends, and no DBRs forreflecting the light in the cavity. The y-direction laser (vertical inthe figure) has a DBR at one end, and a reflective facet at the otherend. Cleaved facet ends reduce the length of the device since the lengthof the DBR sections is omitted.

[0093]FIG. 5c shows a top view of a laser diode with cleaved facets atboth ends. The length of the gain region is fixed by the reflectivity ofthe end facets or the DBRs.

[0094]FIG. 6 shows a laser with two reflective end facets and an OCgrating between the pumped regions. Beneath the OC grating is areflector for reflecting the two downward directed beams back up towardthe surface of the laser. Capturing the downward beams is useful toincrease efficiency.

[0095]FIG. 7a shows a GO laser system with four crossed lasers beingoutcoupled at the same spatial location. Distributed Bragg reflectorsare shown in this example at each end of the individual lasers. Eachlaser has its own OC grating superimposed on the other OC gratings tocreate one virtual grating. (Note that it is possible to overlap thegratings in different planes, having either curved waveguides or makingeach laser totally in a different plane than the others, but this is aless preferred embodiment. HOEs can be used in place of multiplegratings in such a situation.) FIG. 7b shows a close-up of theoutcoupling gratings for the four laser system. There are four gratings,each at a 45 degree angle to the adjacent gratings. The periods of thesegratings need not be the same, which allows the use of multiplewavelengths being outcoupled at the same location (assuming the lasersthemselves are fashioned to output different wavelengths). The periodsof the various gratings are labeled as Lz, Ly, Lzy, and Lyz.

[0096] The structure of the grating outcoupled laser allows for thehigh-level integration of electronic circuitry with the laser device.Many different possible devices can be integrated with the GO laser ofthe present application, including control electronics and photonicintegrated circuits that can be placed on the chip with the laser;electronic integration includes higher level data management electronicssuch as serialization/deserialization, clock generation and recovery, opamps and analog-to-digital and digital-to-analog converters. The GOlaser geometry is also advantageous in that it allows for integrationwith optical waveguides and integration of light routing circuitry onthe chip. Such photonic integration allows optical interconnects betweencircuit elements, integration of optical isolators, wavelength tunablesections, optical modulators, waveguide couplers, waveguide switches,and simultaneous integration of multiple ones of these components.

[0097]FIG. 8 shows an example of a higher level circuit integration onthe same chip as the grating outcoupled laser diode. In this example,laser driver and control electronics are integrated. Din and Din-bar aredifferential data inputs and EN is the chip enable. Internal logic andBIAS control is used to center the data to the inputs of the driver pairand to provide a Vref output. This output is used with Rset to controlthe current source or modulation current Imod which sets the outputcurrent to the grating outcoupled laser diode. The resistor Rext isexternal to minimize the power dissipation capacity, both Rext and Rsetmay be integrated as well.

[0098]FIG. 9 shows the optical waveguide technology routing the lightfrom the laser source to another integrated circuit component, forexample. One end of the laser has a partially transmitting reflector 902which allows some of the light to pass through. This light is guided bya waveguide 904 to another circuit element as desired for any givenapplication. In the example shown, a corner turning mirror 906 is usedto guide the light.

[0099] Other devices can also be integrated on the same chip with theinnovative laser system of the present application. FIG. 10 shows agrating outcoupled laser diode with the OCG 8 between the two reflectors902 at either end of the cavity. One or both reflectors are partiallytransmitting gratings in this example. In this variation, a waveguidecoupler, which may be used for connecting to other integrated devices,is located outside the laser cavity. A location for such an integrateddevice 1002 is shown at either end of the cavity. Of course, otherdevices could also be integrated, for example, modulators or tuningsections.

[0100]FIG. 11 shows another possible variation for integrating addedcomponents (such as a phase adjustor, for example). In this example, tworegions 1002 where added devices may be fabricated on chip are shown. Inthis example, the added components are located within the length of thelaser, possibly requiring the gain region contacts to be moved oraltered to accommodate the added device.

[0101]FIG. 12 shows another embodiment of the present innovations. Inthis variation, a reflective coating 1202 (for example, a dielectricstack or a metal layer) is placed on top of the device, where theoutcoupling grating 1204 is located. This reflective coating causes thelight to be coupled through the bottom of the device. (Note that it isoften useful to allow some light to escape the top of the device in thisdesign, as this light can be used to allow easier wafer level probetesting of the device.) If an outcoupling grating is used without such areflective coating, there is of course light emitted both above andbelow the waveguide due to reflection from the grating. The reflectivecoating causes the top emitted lobe of light to instead be emittedthrough the bottom of the device, so that substantially all lightemitted from the cavity is coupled out through the substrate material,which is a transparent material, usually about 100 microns thick. Thereflective coating can be metallic (such as gold) or it can be adielectric stack for better reflection. Generally, if the light is to beemitted out the bottom of the device, a high reflect (HR) coating isplaced on top of the device to reflect the light downward. Ananti-reflect (AR) coating may be added to the bottom of the device inthis case. Alternatively, if the light is to be emitted out the topsurface, the HR coating may be placed on the bottom of the device, andan AR coating placed on top.

[0102] Causing the light to be emitted through the bottom of the devicehas the advantage of allowing the heat sink to be placed on the top ofthe laser, closer to the locations where heat is generated, increasingefficiency of heat sinking. In such a case, the device is preferablymounted “upside down”, with the reflective coating and DBRs beneath, thesubstrate on top, facing a fiber core for coupling, for example. Thisvariation helps feedback in the cavity and can decrease thresholdcurrent by 50% or more.

[0103] In FIG. 12, the laser device is shown with an outcouplingaperture between the DBRs. An outcoupling grating is shown in thisexample. On top of this grating is located a reflective coating thatdirects outcoupled light down through the substrate.

[0104] The gain region or regions of the present innovations can bemodified to provide added functionality. FIG. 13 shows a DBR laseraccording to an embodiment of the present innovations. The gain region1300 has multiple parts in this example, one on either side of anoutcoupling grating. One part of the gain region is further split intotwo parts, a larger 1302 and a smaller 1304 section. The smaller part,which can be used to more sensitively adjust the current supplied to thegain region, is used for several purposes. It can be used as a finetuning device for the wavelength of the light in the cavity. Byincreasing or decreasing the current, the wavelength can be slightlytuned to some degree. The smaller contact can also be used to modulatethe signal generated in the cavity. By varying the supplied current overtime, the intensity of the emitted light can be varied. This can be usedto modulate the signal by adjusting the current over time to alter theintensity of light, and thus embed a signal in the emitted light. Thesmaller contact is the preferred one to use for such modulation, sinceit will allow faster modulation (due to lower capacitance, etc.).

[0105]FIG. 14 shows a side view of a DBR laser according to anotherembodiment. The laser has a cavity and two reflector gratings 7, one ateach end. Between the reflector gratings are the gain region 1402 andthe outcoupling grating 8.

[0106] The waveguide 3 in the laser may be made from multiple layersincluding an active and guiding layer containing the junction betweenthe p and n type semiconductor materials. For example, this region mightcomprise a sequence of one or more barrier layers alternating withquantum well layers.

[0107] In the embodiment shown in the figure, the quantum wells 1404 aremade thinner beneath the reflector gratings 7 and beneath theoutcoupling grating 8. This results in a less lossy, more transparentdevice. The larger bandgap in the thinner quantum well regions (i.e., inthe unpumped regions) means less absorption of photons in the cavitybecause higher photon transition energy is required. This lowersinternal loss, increases efficiency, and lowers the required thresholdcurrent for the device.

[0108] The quantum well thickness, by controlling the requiredtransition energy, affects the wavelength of the photons that will lasein the cavity. This allows large scale tunability of the device duringfabrication by controlling quantum well formation.

[0109] Quantum wells are fabricated at different thicknesses usingselective growth of epitaxial layers. This selective growth phase ofprocessing can also be used to simultaneously improve the performance ofintegrated components such as electro-absorption modulators, whichbenefit from quantum well structures that can be made more transparent(or have a higher photon transition energy). The integrated devices arefabricated at the same time and during the same set of processes as thelasers themselves, and selective growth can be used on them withoutsignificant process cost added.

[0110]FIG. 15 shows another embodiment. In this variation, afractional-spherical lens 1502 made of an optical polymer is formed onthe outcoupling grating 1504 (by a microjet process, for example). Thislens acts to make different wavelengths of light (which may exit the OCGat different angles than the normal, or from one another) converge to becoupled into a fiber mode, for example. Since the index of refraction ofthe material outside the lens is lower than the index of refraction ofthe lens, light rays passing through the lens to the lower indexmaterial will be refracted away from the normal to the surface of thelens at that point. Depending on the index difference, the refractioncan be great or small. This element can be used to allow design marginin the size of the outcoupling grating. With the added lens, theoutcoupling grating can be made larger, (50 microns, for example), whichis useful in high power applications. Normally, an outcoupling gratingof 10 micron size can outcouple light in the hundreds of milliwatt rangeor higher, but with larger outcoupling gratings light of higher power,well above a watt, can be outcoupled and confined to a fiber mode.

[0111] Other types of intermediate lens devices (gratings, HOEs, etc.)can also be used at this location to aid in coupling light from theoutcoupling grating into a fiber core (for example). Such devices can bemade on chip, or be separate devices added after processing is complete.

[0112]FIG. 15 shows a sample embodiment of the device. This embodimentcomprises a DBR laser as described previously in this application, withoutcoupling grating 1504 located between the two reflector gratings1506. Gain regions 1508 are on either side of the OCG. A lens 1502 whichis partly spherical in shape is formed on the outcoupling grating.

[0113] Definitions

[0114] Following are short definitions of the usual meanings of some ofthe technical terms which are used in the present application. (However,those of ordinary skill will recognize whether the context requires adifferent meaning.) Additional definitions can be found in the standardtechnical dictionaries and journals.

[0115] Chirped Grating: a grating whose period varies over its area.;

[0116] DBR: Distributed Bragg Reflector;

[0117] DFB: Distributed Feedback (Laser);

[0118] DWDM: Dense Wavelength Division Multiplexing;

[0119] GO: Grating Outcoupled (Laser);

[0120] HOE: Holographic Optical Element;

[0121] LED: Light Emitting Diode;

[0122] LD: Laser Diode

[0123] Modulate: any controlled variation of the signal, in intensity orfrequency or phase;

[0124] OCG:Outcoupling Grating;

[0125] VCSEL:Vertical Cavity Surface Emitting Laser;

[0126] Waveguide: a structure which guides the propagation of photons,usually by internal reflection.

[0127] The following background publications provide additional detailregarding possible implementations of the disclosed embodiments, and ofmodifications and variations thereof, and the predictable results ofsuch modifications. All of these publications are hereby incorporated byreference: “Surface Emitting Semiconductor Lasers and Arrays,” Ed. Evansand Hammer, Academic Press, 1993; “Research Toward Optical Fibertransmission Systems Part 1,” Proc. IEEE, 61, 1703-1751, Dec. 1973;“Optimized Couplers Between Junction lasers and Single Mode Fibers,”Hammer, Neil, RCA laboratories, Princeton, N.J., Final Report, Aug. 31,1981-Jan. 31, 1983; “Observations and Theory of High Power Butt Couplingto LiNbO₃ -type waveguides,” Hammer and Neil, IEEE J. QuantumElectronics, QE-18, 1751-1758, October 1982; “Laser Diode End FireCoupling into Ti:LiNbO₃ waveguides,” Appl, Optics, 18, 2536-2537, August1979.

[0128] Modifications and Variations

[0129] As will be recognized by those skilled in the art, the innovativeconcepts described in the present application can be modified and variedover a tremendous range of applications, and accordingly the scope ofpatented subject matter is not limited by any of the specific exemplaryteachings given.

[0130] The gratings used for reflectors in many embodiments mentioned inthis application can vary in form. One example includes a grating thatwill reflect two wavelengths by use of a sampled grating—two gratingssuperimposed on one another, one coarser than the other. Other examplesinclude the use of chirped gratings, blazed gratings, or variable pitchgratings.

[0131] The outcoupling element in the disclosed embodiments can be agrating as described, or a holographic optical element, a lens, or anyother outcoupling device. For example, a beam splitter properlypositioned can also couple light out of the cavity. Blazed gratings mayalso be used. Circular gratings (which produce a fanned light output)can also be used in some applications.

[0132] The waveguide structure can be of different forms, for example, aridge waveguide or a buried heterojunction.

[0133] Note that though the examples given show lasers with two separatecontacts for the gain region, one on either side of the outcouplinggrating, the present innovations also contemplate lasers with only onecontinuous gain region located to one side of the OC grating, orcontinuous gain regions that yet span both sides of the OC grating.

[0134] The innovative structures can also include DFB gratings ratherthan DBRs to reflect cavity mode light.

[0135] None of the description in the present application should be readas implying that any particular element, step, or function is anessential element which must be included in the claim scope: THE SCOPEOF PATENTED SUBJECT MATTER IS DEFINED ONLY BY THE ALLOWED CLAIMS.Moreover, none of these claims are intended to invoke paragraph six of35 USC section 112 unless the exact words “means for” are followed by aparticiple.

What is claimed is:
 1. A semiconductor laser device, comprising: awaveguide having separate first order reflector gratings at both ends ofsaid waveguide; an outcoupling location positioned between said gratingson said waveguide, connected to couple light out of said waveguide. 2.The device of claim 1, wherein said gratings are distributed Braggreflectors.
 3. The device of claim 1, wherein said light is coupled outat an angle other than the normal to the surface of said device.
 4. Thedevice of claim 1, wherein said outcoupling location comprises a firstorder grating which couples light out of said waveguide.
 5. The deviceof claim 1, further comprising a reflective surface positioned atop thedevice at said outcoupling aperture to reflect light downward throughthe bottom of said device.
 6. The device of claim 1, wherein saidoutcoupling location comprises a holographic optical element.
 7. Asemiconductor laser device, comprising: a waveguide structure havingfirst and second reflectors, one at either end of said waveguide; afirst set of electrodes connected to pump a first gain region portion ofsaid waveguide structure adjacent to said first reflector; a second setof electrodes connected to pump a second gain region portion of saidwaveguide structure adjacent to said second reflector; an outcouplingaperture positioned between said first and second gain region portionson said waveguide structure, connected to couple light out of saidwaveguide structure.
 8. The device of claim 7, wherein at least one ofsaid reflectors is a facet with a reflective coating.
 9. The device ofclaim 7, wherein said first set of electrodes comprises two parts, oneof said parts being used to modulate said device.
 10. The device ofclaim 7, wherein said outcoupling aperture comprises a first ordergrating with a non-circular footprint.
 11. The device of claim 7,wherein said outcoupling aperture is matched to the mode of a fiberwaveguide.
 12. The device of claim 7, wherein said outcoupling aperturecomprises a grating having a layer of material thereon, said layerlimiting the number of photons exiting said aperture.
 13. The device ofclaim 7, wherein said device is integrated with other optical elementson a single semiconductor substrate.
 14. A semiconductor laser device,comprising: a cavity having reflectors at either end and an outcouplingaperture connected to outcouple light from said cavity; a gain region ofsaid cavity located between said reflectors, said gain region having afirst portion on one side of said outcoupling aperture and a secondportion on the opposite side of said outcoupling aperture.
 15. Thedevice of claim 14, wherein said first portion of said gain region hastwo parts, one of said parts being used to modulate said device.
 16. Thedevice of claim 14, wherein said outcoupling aperture comprises a beamsplitter which outcouples light by reflecting it in a directionperpendicular to the surface of said device.
 17. The device of claim 14,further comprising a dielectric coating on said outcoupling aperture,said coating reducing the number of photons exiting said outcouplingaperture.
 18. The device of claim 14, further comprising a reflectivelayer on said outcoupling aperture which reflects light downward throughthe bottom of said device.
 19. The device of claim 14, wherein saidoutcoupling aperture comprises a grating with a circular footprint. 20.The device of claim 14, wherein said reflectors are distributed Braggreflectors each having a grating strength, and wherein said gratingstrength for at least one of said reflectors varies laterally andlongitudinally with respect to said cavity.
 21. A semiconductor lasersystem, comprising: a cavity having reflectors at either end and anoutcoupling aperture connected to outcouple light from said cavity, saidoutcoupling aperture located between said reflectors; a gain region ofsaid cavity located between said reflectors; a reflective layerpositioned on said outcoupling aperture.
 22. The system of claim 21,wherein said gain region has multiple parts, one of which has a variablecurrent for modulating the output light.
 23. The system of claim 21,wherein said reflective layer reflects light downward through the bottomof said cavity.
 24. The system of claim 21, wherein said gain region hasmultiple parts, one of which has a variable current for tuning thewavelength of the output light.
 25. The system of claim 21, whereinlight is coupled out of the laser normal to the surface of the laser.26. The system of claim 21, wherein said system is integrated on asingle semiconductor substrate with other optical elements.
 27. Asemiconductor laser system, comprising: a cavity having reflectors ateither end and an outcoupling aperture connected to outcouple light fromsaid cavity, said outcoupling aperture located between said reflectors;a gain region of said cavity located between said reflectors; whereinsaid gain region is divided into a plurality of sections, one of saidsections being connected to modulate said light.
 28. The system of claim27, wherein at least one of said reflectors is a distributed Braggreflector.
 29. The system of claim 27, wherein said outcoupling aperturecomprises a grating which couples light out of the laser at an angleother than normal to the surface of said laser.